308 CRETACEOUS PELECYPODA 



Other cliaructers will be seen specified under the heads of the different families. 

 I may only notice that in the same manner, as the Teiqoniid^ on the one side 

 connect this order with the AstartidjE, so do the mytyliform species of Barbatia 

 and its allies, (especially characteristic in the eocene), indicate a transition to 

 the next order, the 3IYTILACEA. 



In a geological point of view, the ABCACEA are an important order, for of 

 the four families every one is represented already in the Silurian deposits, but their 

 greatest development falls into different periods ; that of the Trigoniid^ into the 

 Jurassic, that of the Aecid^ in the cretaceous, and that of the Nuculanidm and 

 NucvLiDM either in the tertiary or in the present epoch. 



The number of recent species belonging to the order is,- even supposing consi- 

 derable additions have been made recently-, barely 400, while that of the fossil 

 species is at least three times as large. 



XXXIII. Family— TRIG ONIIBM. 



The animals of Trigonia (group Peetinatce, see p. 311), have the mantle- 

 margins entirely open and fringed, the edges between each of the larger papillae 

 being provided with small capsules, Avhich appear to be eyes,* resembling those of 

 JPecteu ; the foot is rather large, geniculate, somewhat expanded, anteriorly produced 

 and with slightly serrated edges, some lateral grooves on it have been suggested 

 as representing byssal grooves, but no byssus has been observed; the palpi are 

 small, two on each side, somewhat unequal, the upper apparently a little smaller 

 than the lower; gills rather large, sub- triangular, two on each side, anteriorly 

 and posteriorly grown together ; no special siphons are present. 



The shells are mostly elongately, or broadly and ovately, sub-trigonal, with two 

 or three cardinal hinge-teeth in one and three or four in the other valve, they are 

 either smooth or, more generally, partially transversally sulcated ; there are two 

 muscular impressions, each, and especially the posterior, is superseded by a 

 distinct small pit which is caused by the retractile muscles of the foot. In the 

 Trujonice. the posterior of these muscles appears to be the stronger one, while 

 in the Astartidje the anterior is the much stronger one. The pallial impression 

 is always entire, the ligament external, and the internal structure either pearly 

 or calcareous. 



As regards the anatomy of the animals the Trigonkc are intei-mediate between 

 the NucuLiD^ and the Arcid^, clearly showing most marked relations to the 

 former. The shells, on the other hand, exhibit a great similarity to those of the 

 Astartidjb and Crassatellid^, both in the general form as well as in the distri- 

 bution of the hinge-teeth, though this relation is not so apparent in the recent as 

 it is in the fossil forms, the former exhibiting in the striation of the hinge-teeth 

 considerable affinities to the hinge of the Nvculibm. 



* The most recent and best account of the aiuitomy ol' ii recent species, probably T. mari/arituceit, is publislied by 

 Dr. E. Selenka in Malacozool. Bk-tter, xv, p. 68. 



